1. Field of the Invention
The invention relates to distributed Bragg reflectors (DBRs), and particularly to a DBR manufactured with a compositional grading scheme and exhibiting low resistance.
2. Description of the Related Art
The need for increasing data-transfer capacity has led to the development of high-performance optoelectronic components, including vertical-cavity, surface-emitting lasers (VCSELs). VCSELs consist of an active region located between two highly-reflective mirrors, as shown in FIG. 1. Conventionally, these mirrors consist of alternating layers of semiconductor materials, each with a different refractive index, and of a specific thickness. This structure is known as a distributed Bragg reflector (DBR). Typically, VCSELs with semiconductor mirrors have DBRs with approximately 20 pairs of alternating layers within each reflector. The simplest form of DBR includes abrupt interfaces between the layers within the mirror. Abrupt interfaces involve changing of materials within a short distance from one material to another. For example, the DBRs shown in FIG. 1 show abrupt interfaces (or changes in material) between each of the mirror layers. The highly-reflective DBR results from the summation of reflections from each of the interfaces between the two layers. By choosing the thickness of each of the DBR layers to be an odd multiple of a quarter wavelength (□) in the material (□/4n), the summed reflections will add in phase to realize a highly reflective mirror. The most popular version of the DBR is the use of alternating layers of III-V semiconductors, such as AlxGa1-xAs and AlyGa1-yAs (x≠y), for use in GaAs-based VCSELs operating a wavelengths such as 850 nm, 980 nm, or 1300 nm.
To realize high-performance VCSELs, the DBRs should be optimized to have both good electrical and optical properties. An optimized DBR will have a high optical reflectance, typically >99.9%, while having low optical loss. However, good electrical performance, e.g., having low electrical resistance, often requires changes in the DBR structure that will reduce the optical performance of the mirror. Thus, realizing a high performance VCSEL often involves a compromise between the electrical and optical performance of the mirror. One method to dramatically improve the electrical behavior, while keeping the doping low and only affecting the optical performance in a minor way, is to compositionally grade the materials at the interface between the two DBR materials. The grading scheme acts to reduce the device resistance to current (charge carrier) flow which translates into enhanced device performance characteristics, including reduced threshold voltages, higher output powers, and improved temperature-dependent behaviors. With the AlxGa1-xAs material system, this can be achieved due to the near lattice-matched conditions present for all compositions of AlxGa1-xAs on GaAs. The compositional grading reduces, or even eliminates, the magnitude of the energy band discontinuities present in the DBR structure. These discontinuities act as energy barriers preventing the flow of injected charge carriers, either electrons or holes, through the mirror, thus adding to the overall resistance of the DBR.
The functional shape of the compositional grading can also affect the resistance of the DBRs. Several different grading schemes, including linear and parabolic, have been successfully demonstrated resulting in low-resistance DBRs. [see references 1, 2, 3, below]. The linear grading scheme is simpler to implement and is commonly used in VCSELs. However, the more complicated parabolic grading scheme is also commonly used to flatten the valence band and further optimize device performance. Typically, linear grading schemes are employed in n-type DBRs, which use electrons as the charge carriers, while parabolic grading schemes are used in p-type mirrors, which employ holes as the charge carriers.
Several different ways of realizing compositional grading have been demonstrated. The method used depends on the growth apparatus used in the formation of the VCSEL structure. These include analog grading schemes realized using metal-organic, chemical vapor deposition (MOCVD) and digital superlattice grading schemes realized using molecular beam epitaxy (MBE). The method proposed here is realized with a multi-source configuration using MBE that results in a digital alloy-grading scheme providing a novel method for realizing low-resistance DBRs.